In cryptography, a cipher is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.
In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is larger than or equal to the size of the message being sent. In this technique, a plaintext is paired with a random secret key. Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition.
In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters, pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution process to extract the original message.
In cryptography, a transposition cipher is a method of encryption which scrambles the positions of characters (transposition) without changing the characters themselves. Transposition ciphers reorder units of plaintext according to a regular system to produce a ciphertext which is a permutation of the plaintext. They differ from substitution ciphers, which do not change the position of units of plaintext but instead change the units themselves. Despite the difference between transposition and substitution operations, they are often combined, as in historical ciphers like the ADFGVX cipher or complex high-quality encryption methods like the modern Advanced Encryption Standard (AES).
In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.
The Vigenère cipher is a method of encrypting alphabetic text where each letter of the plaintext is encoded with a different Caesar cipher, whose increment is determined by the corresponding letter of another text, the key.
An autokey cipher is a cipher that incorporates the message into the key. The key is generated from the message in some automated fashion, sometimes by selecting certain letters from the text or, more commonly, by adding a short primer key to the front of the message.
In cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers.
In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it. This process prevents the loss of sensitive information via hacking. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. Ciphertext is not to be confused with codetext because the latter is a result of a code, not a cipher.
The Lorenz SZ40, SZ42a and SZ42b were German rotor stream cipher machines used by the German Army during World War II. They were developed by C. Lorenz AG in Berlin. The model name SZ was derived from Schlüssel-Zusatz, meaning cipher attachment. The instruments implemented a Vernam stream cipher.
A straddling checkerboard is a device for converting an alphanumeric plaintext into digits whilst simultaneously achieving fractionation and data compression relative to other schemes using digits. It also is known as a monôme-binôme cipher.
Kryptos is a sculpture by the American artist Jim Sanborn located on the grounds of the Central Intelligence Agency (CIA) headquarters, the George Bush Center for Intelligence in Langley, Virginia.
The known-plaintext attack (KPA) is an attack model for cryptanalysis where the attacker has access to both the plaintext and its encrypted version (ciphertext). These can be used to reveal secret keys and code books. The term "crib" originated at Bletchley Park, the British World War II decryption operation, where it was defined as:
A plain language passage of any length, usually obtained by solving one or more cipher or code messages, and occurring or believed likely to occur in a different cipher or code message, which it may provide a means of solving.
In cryptography, the ADFGVX cipher was a manually applied field cipher used by the Imperial German Army during World War I. It was used to transmit messages secretly using wireless telegraphy. ADFGVX was in fact an extension of an earlier cipher called ADFGX which was first used on 1 March 1918 on the German Western Front. ADFGVX was applied from 1 June 1918 on both the Western Front and Eastern Front.
Gilbert Sandford Vernam was a Worcester Polytechnic Institute 1914 graduate and AT&T Bell Labs engineer who, in 1917, invented an additive polyalphabetic stream cipher and later co-invented an automated one-time pad cipher. Vernam proposed a teleprinter cipher in which a previously prepared key, kept on paper tape, is combined character by character with the plaintext message to produce the ciphertext. To decipher the ciphertext, the same key would be again combined character by character, producing the plaintext. Vernam later worked for the Postal Telegraph Company, and became an employee of Western Union when that company acquired Postal in 1943. His later work was largely with automatic switching systems for telegraph networks.
In cryptography, a classical cipher is a type of cipher that was used historically but for the most part, has fallen into disuse. In contrast to modern cryptographic algorithms, most classical ciphers can be practically computed and solved by hand. However, they are also usually very simple to break with modern technology. The term includes the simple systems used since Greek and Roman times, the elaborate Renaissance ciphers, World War II cryptography such as the Enigma machine and beyond.
In cryptography, the M-209, designated CSP-1500 by the United States Navy is a portable, mechanical cipher machine used by the US military primarily in World War II, though it remained in active use through the Korean War. The M-209 was designed by Swedish cryptographer Boris Hagelin in response to a request for such a portable cipher machine, and was an improvement of an earlier machine, the C-36.
The pigpen cipher is a geometric simple substitution cipher, which exchanges letters for symbols which are fragments of a grid. The example key shows one way the letters can be assigned to the grid.
BATCO, short for Battle Code, is a hand-held, paper-based encryption system used at a low, front line level in the British Army. It was introduced along with the Clansman combat net radio in the early 1980s and was largely obsolete by 2010 due to the wide deployment of the secure Bowman radios. BATCO consists of a code, contained on a set of vocabulary cards, and cipher sheets for superencryption of the numeric code words. The cipher sheets, which are typically changed daily, also include an authentication table and a radio call sign protection system.
The Aristocrat Cipher is a type of monoalphabetic substitution cipher in which plaintext is replaced with ciphertext and encoded into assorted letters, numbers, and symbols based on a keyword. The formatting of these ciphers generally includes a title, letter frequency, keyword indicators, and the encoder's nom de plume. The predecessor to these ciphers stems from the Caesar Cipher around 100. The Aristocrat Cipher also used a transposition of letters to encrypt a message.
